1. Field of the Invention
The present invention relates generally to the manufacture of semiconductor nanowires and, more specifically, to the manufacture of silicon nanowires on conducting substrates, and to selected applications and end uses of the resulting structures.
2. Description of the Prior Art
Nanostructures, which include but are not limited to nanofibers, nanowires and nanocrystals, have potential uses in a variety of fields including electrical, chemical and optical applications. Likewise, these unique structures appear to offer great potential advantages in the biomedical field. Researchers at Texas Christian University and others are presently developing semiconductor nanowires, specifically silicon (Si) nanowires, for applications relevant to biomaterials in general, and orthopedics in particular.
Nanomaterials are currently known to be used in a variety of applications in the fields mentioned above. As an example of the current state of the art, U.S. Pat. No. 's7,087,833; 7,087,832; and 7,083,104, all assigned to Nanosys, Inc., of Palo Alto, Calif., teach the manufacture of nanostructure and nanocomposite materials, many of which have potential electronic end applications. For example, U.S. Pat. No. 7,083,104, teaches the use of semiconductor devices which incorporate thin films of nanowires for applications in the field of macroelectronics. The teachings are specifically directed to the manufacture of an adjustable phase shifter, as well as to applications involving a radio frequency identification tag. In applications such as these, electrical contacts or switches usually include a transistor that is formed by a film of nanowires.
Silicon nanowires also have potential technological benefits in the field of biomaterials, and specifically in the field of orthopedics. For example, such nanowire structures could be transferred to existing tissue engineering platforms. There is a long standing need to create to create biocompatible devices which would aid in bone growth. Phosphates are a component of numerous biomaterials used for drug delivery and tissue engineering. For example, calcium phosphates, in the form of powders and solid scaffolds, have been used to efficiently regenerate or promote bone formation. Techniques for promoting the subsequent growth of calcified mineral phases on nanowire surfaces offer the possibility of producing bone-like materials with a wide range of end uses. Similarly, the surface modification of such nanowires with orthopedically relevant anti-osteoporodic drug moieties or anti-bacterial components offer a variety of technological benefits in various medical applications.
One prior art technique for depositing bone-like layers of calcium phosphate on substrates which does not involve nanowires is that which is described in U.S. Pat. No. 6,569,292, “Method and Device For Forming A Calcium Phosphate Film On A Substrate”, issued May 27, 2003, to Jeffery L. Coffer. This patent teaches a technique for applying an inorganic film to an isolating substrate, such as silicon substrate, for intended use in the body. In particular, the patent teaches a technique for forming a calcium phosphate film on a substrate for applications involving the dispensing of medicinally active agents inside a living being. The silicon substrate is first exposed to calcium phosphate to form a coating. An electrode is then positioned a predetermined distance from the coating on the substrate and a current is generated between the electrode and the substrate. The resulting current creates a spark which forms a fixed calcium phosphate film having a desired morphology and thickness within the selected region of the substrate.
While this technique provides a very simple way to produce a calcium phosphate film on a silicon substrate, the growth of calcified mineral phases on nanowire surfaces would result in structures offering increased reactivity or efficiency for many applications due primarily to the increased surface area of the substrates involved.
Several different techniques are known in the prior art for producing nanowires of the general type under consideration. These include template-assisted synthesis, laser ablation, chemical vapor deposition (CVD), electrochemical deposition, and the vapor-liquid-solid (VLS) technique. Using these different techniques, a large variety of semiconducting nanowires have been produced including indium phosphate, gallium nitride, germanium and silicon nanowires. Most of the recent successful semiconducting nanowire growth has been based on the VLS technique.
The VLS method was developed in the 1960's and has seen various improvements in recent years. Some of these improvements involve the metallic nanoparticles used in the process, i.e., the metallic eutectic particles, generally gold, aluminum, iron, cobalt, manganese or silver used to catalyze silicon nanowire growth. Gold is probably the most common metallic eutectic particle utilized, although aluminum is a standard metal in silicon process lines and provides an alternative for silicon nanowire growth. To briefly describe the VLS method, nanosized metallic particles are first formed on a substrate in a nucleation stage. These particles can be formed, for example, by ablation or by annealing a very thin metallic film above the eutectic temperature in order to break it into discrete islands. Then the source material carrier gas (typically SiH4 or SiCl4) is introduced into a chamber maintained above the eutectic temperature. The background pressure is used to control the catalyst size, and the temperature of the tube has to be adjusted in order to maintain the catalyst in the liquid state. Next, the silicon diffuses through the catalyst droplets. When the eutectic alloy has become saturated, silicon precipitates at the liquid-solid interface. Anistropic growth continues while the gas flow is maintained, resulting in the elongation and growth of the nanowire.
While the above technique can be used to produce silicon nanowires of high purity, the process is complicated, requires fairly extensive laboratory equipment, and requires careful monitoring of several different parameters.
A need exists, therefore, for a process for producing semiconductor nanowire, and specifically silicon nanowires which process is simpler in design and more economical in practice.
A need also exists for a process for promoting the high surface area growth of semiconductor nanowires on a conducting substrate and which would allow transfer to existing tissue engineering platforms.
A need also exists for an improved process to promote the subsequent uniform growth of calcified mineral phases on such nanowire surfaces.
A need also exists for an improved process for the control of phase identity of such mineral films through proper surface modification of the nanowires.
A need also exists for a process for surface modification of nanowires so produced, with orthopedically relevant drugs or medicinal components.
A need also exists for a process for producing such surfaces capable of such diverse uses as facilitating the adsorption and mediating the proliferation of mesenchymal stem cells, such cells being capable of differentiation into bone forming osteoblast cells.